Heretofore, as high polymer materials having rubber-like elasticity, those obtained by formulating a crosslinking agent, a reinforcing material, and the like in various rubbers, such as natural and synthetic rubbers, and crosslinking the resulting compositions at high temperature and high pressure have been used broadly. However, such rubbers require prolonged crosslinking and molding under high-temperature, high-pressure conditions, thus being poor in processability. Moreover, the crosslinked rubbers do not exhibit thermoplasticity so that, unlike thermoplastic resins, recycle molding is generally infeasible. For this reason, recent years have seen the development of several thermoplastic elastomers which may be easily processed into shaped articles by utilizing the universal melt-molding techniques such as hot-press molding, injection molding and extrusion molding as it is the case with ordinary thermoplastic resins.
Additionally, a plasticized polyvinyl chloride compound has been generally used as a material having flexibility. Indeed it has been used for various uses as a flexible material at room temperature, but it is demanded to be substituted with other alternative materials in view of recent request of PVC elimination. Therefore, a thermoplastic elastomer compositions have been used as alternatives thereof.
As such thermoplastic elastomers, several polymers in the olefin, urethane, ester, styrenic, and vinyl chloride series have been developed and are on the market today.
Among these polymers, styrenic thermoplastic elastomers are highly flexible and exhibit satisfactory rubber-like elasticity at atmospheric temperature. As such styrenic thermoplastic elastomers, styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS), and the corresponding hydrogenated elastomers such as styrene-ethylenebutylene-styrene block copolymer (SEBS) and styrene-ethylenepropylene-styrene block copolymer (SEPS) are developed. However, these block copolymers are inadequate in permanent compression set characteristics.
Meanwhile, as a thermoplastic elastomer having good flexibility and exhibiting good rubber-like elasticity at atmospheric temperature, and, furthermore, having excellent gas barrier and sealing properties, an isobutylene block copolymer comprising a polymer block composed predominantly of isobutylene and a polymer block composed predominantly of an aromatic vinyl compound is known. However, this isobutylene block copolymer has also proved unsatisfactory in the degree of compressive deformation on heating (permanent compression set) and in rubber-like elasticity at high temperature.
Also known is a thermoplastic polymer composition comprising a crosslinking product of an isobutylene block copolymer component containing a polymer block composed predominantly of isobutylene and a rubber component (WO98/14518). This composition is improved in permanent compression set characteristics, but it is still unsatisfactory.